Acrolein is a key intermediate for the synthesis of methylmercaptopropionaldehyde and of methionine, a synthetic aminoacid used as an animal feed supplement, which has emerged as a substitute for fishmeal. Acrolein is also a non-isolated synthetic intermediate of acrylic acid and acrylonitrile for which the importance of their applications and its derivatives is known. Acrolein also leads, via reaction with methyl vinyl ether then hydrolysis, to glutaraldehyde, which has many uses in leather tanning, as a biocide in oil well drilling and during the treatment of cutting oils, and as a chemical sterilant and disinfectant for hospital equipment. Acrolein also leads to pyridine or glutaraldehyde.
Acrylic acid is a compound that is used essentially as polymerization monomer or comonomer for the manufacture of a very broad range of final products. These final products are manufactured by polymerization of the acid or of the derivatives of this acid, in the ester (polyacrylates) or amide (polyacrylamides) form. A very important outlet for acrylic acid is the manufacture of superabsorbents, in which a partially neutralized (mixture of acrylic acid and sodium acrylate or acrylates of other cations) acrylic acid is polymerized, or else acrylic acid is polymerized and the polyacrylic compound obtained is partially neutralized. These polymers are used as is or as copolymers in fields as varied as hygiene, detergents, paints, varnishes, adhesives, paper, textiles, leather, and the like.
Acrolein and/or acrylic acid are produced industrially by oxidation of propylene using oxygen or an oxygen-comprising mixture in the presence of catalyst systems based on mixed oxides. This reaction is generally carried out in the gas phase and generally in two stages to give acrylic acid: the first stage carries out the substantially quantitative oxidation of the propylene to give an acrolein-rich mixture, in which acrylic acid is a minor component, and then the second stage carries out the selective oxidation of the acrolein to give acrylic acid.
The reaction conditions of these two stages, carried out in two multitubular reactors in series or in a single reactor comprising the two reaction stages in series, are different and require catalysts suited to the reaction; however, it is not necessary to isolate the acrolein from the first stage during this two-stage process.
The starting materials used for acrolein or acrylic acid production result from oil or natural gas and consequently the acrolein or acrylic acid are formed from a non-renewable fossil carbon starting material. In the context of the commitments of the majority of industrialized countries to reduce emissions of greenhouse gases, it appears particularly important to manufacture novel products based on a renewable starting material, contributing to reducing the environmental effects and global warming potential.
Glycerol is derived from plant oils in the production of biodiesel fuels or oleochemicals such as fatty acids or fatty alcohol or fatty esters. Glycerol is one of the raw materials envisaged as a substitute for propylene, glycerol being able to be subjected to a catalytic dehydration reaction in order to produce acrolein. Such a process makes it possible to thus respond to the concept of green chemistry within a more general context of protecting the environment. There are also many possible ways to access to renewable glycerol, for example by fermentation of sugars or by hydrogenolysis reactions.
This process is highly analogous to the synthetic process starting from propylene insofar as the intermediate product, acrolein, resulting from the first stage is the same and insofar as the second stage is carried out under the same operating conditions.
However, the reaction of the first stage of the process of the invention, the dehydration reaction, is different from the reaction for the oxidation of propylene of the usual process. The dehydration reaction, performed in the gas phase, is carried out using different solid catalysts from those used for the oxidation of propylene. The acrolein-rich effluent resulting from the first dehydration stage, intended to feed the second stage of oxidation of acrolein to give acrylic acid, comprises a greater amount of water and in addition exhibits substantial differences as regards by-products resulting from the reaction mechanisms involved.
Numerous catalyst systems have already been the subject of studies for the dehydration reaction of glycerol to acrolein.
U.S. Pat. No. 5,387,720 describes a process for producing acrolein by dehydration of glycerol, in liquid phase or in gas phase, at a temperature ranging up to 340° C., over acidic solid catalysts that are defined by their Hammett acidity. The catalysts must have a Hammett acidity below +2 and preferably below −3. These catalysts correspond, for example, to natural or synthetic siliceous materials, such as mordenite, montmorillonite and acidic zeolites; supports, such as oxides or siliceous materials, for example alumina (Al2O3), titanium oxide (TiO2), covered by monobasic, dibasic or tribasic inorganic acids; oxides or mixed oxides such as gamma-alumina, ZnO/Al2O3 mixed oxide, or else heteropolyacids. The use of these catalysts would make it possible to solve the problem of formation of secondary products generated with the iron phosphate type catalysts described in the document FR 695,931.
According to Application WO 06/087084, the strongly acidic solid catalysts whose Hammett acidity H0 is between −9 and −18 have a strong catalytic activity for the dehydration reaction of glycerol to acrolein and are deactivated less quickly.
In the document WO 09/044,081 it has been proposed to carry out the reaction for dehydration of glycerol in the presence of a catalyst system comprising oxygen, iron, phosphorus, and one or more elements chosen from alkali metals, alkaline-earth metals, Al, Si, B, Co, Cr, Ni, V, Zn, Zr, Sn, Sb, Ag, Cu, Nb, Mo, Y, Mn, Pt, Rh and the rare earths La, Ce, Sm.
The document WO 09/128,555 describes a process for preparing acrolein by dehydration of glycerol in the presence of a catalyst comprising mainly a compound in which protons in a heteropolyacid are exchanged at least partially with at least one cation selected from elements belonging to Group 1 to Group 16 of the Periodic Table of Elements.
In the document WO 10/046,227 the dehydration of glycerol is performed in the presence of a catalyst system comprising oxygen, phosphorus and at least one metal chosen from vanadium, boron or aluminium.
However, the catalysts recommended in the prior art for producing acrolein from glycerol generally lead to the formation of by-products such as hydroxypropanone, propanaldehyde (called also propanal), acetaldehyde, acetone, addition products of acrolein to glycerol, polycondensation products of glycerol, cyclic glycerol ethers, and also phenol and polyaromatic compounds which are the source of the formation of coke on the catalyst and therefore of its deactivation.
The presence of the by-products in acrolein, especially propanal, poses numerous problems for the separation of acrolein and requires separation and purification steps which lead to high costs for the recovery of the purified acrolein. Furthermore, when acrolein is used for producing acrylic acid, the propanal present may be oxidized to propionic acid, which is difficult to separate from acrylic acid, especially by distillation. Indeed, propanal and propionic acid have their boiling points of respectively 49° C. and 141° C. which are very close to boiling points of 53° C. and 141° C. respectively of the objective compounds of acrolein and acrylic acid. The same problem occurs when acrolein is used to make methionine, or acetals or acrylonitrile, since the boiling points of acrylonitrile and propionitrile are respectively 77° C. and 97° C. These problems exist for the two means of production of acrolein/acrylic acid or acrylonitrile—from propylene or from glycerol—since propanal results as a by-product in the glycerol dehydration and in the propylene oxidation, but propanal is in a greater amount in the case of glycerol, probably owing to a lower selectivity of the reaction of dehydration of glycerol.
These impurities that are present greatly reduce the field of application of the acrolein and acrylic acid produced by dehydration of glycerol. In particular, impurities such as non-polymerizable saturated compounds can be particularly troublesome in the final application by modifying the properties of the finished product, by conferring toxic or corrosive properties on the finished product or by increasing polluting organic discharges during the stages of manufacture of the acrylic polymer and/or of the finished product.
Consequently, there is a need for an acrylic acid which meets all the above-mentioned constraints, both upstream, that is to say an acrylic acid essentially based on a non-fossil natural carbon source, and downstream, that is to say an acrylic acid which meets quality standards allowing it to be used in the manufacture of a broad range of technical polymers, without, however, requiring a sophisticated and therefore expensive purification.
To meet this need, it has already been proposed, for example in WO 09/044,081, to place two active catalyst beds in series: the gaseous reaction mixture containing glycerol is sent to a first catalyst in contact with which the dehydration reaction of glycerol is at least partially carried out generally resulting in secondary compounds such as propanal. The reaction mixture thus obtained is passed over a second catalyst bed such as a doped catalyst system based on iron phosphate, on which the dehydration reaction of unreacted glycerol may continue at the same time as the conversion of propanal to acrolein. The acrolein obtained according to this embodiment contains a minimized amount of propanal, which widens its field of application and facilitates obtaining high purity acrylic acid. However, it was observed that such catalysts lead to a rapid plugging of the facility due to the formation of products like tar.
The configuration of two active catalyst beds in series to limit the presence of propanal in acrolein has also been described in the process of WO 10/046,227 using a catalyst system based on a mixed oxide of phosphorus and vanadium. However, these catalysts are less active at moderate temperature.
The document WO 10/074,177 relates to a method for preparing acrylic acid from a composition containing acrolein and propanal by gas phase reaction using a solid catalyst comprising at least Mo and V as essential components. In this gas phase reaction, acrolein is converted to acrylic acid and propanal is converted to acrylic acid and to propionic acid. With this acrolein oxidation catalyst, propanal is converted at a similar rate as acrolein, and propanal is mainly converted to propionic acid. The conversion of propanal into acrylic acid is very low (3%), and the acrylic acid thus obtained contains a very high propionic acid content and it has to be purified by crystallization to remove propionic acid.
Also, it has been proposed to remove propionic acid from an acrylic acid stream, in particular from an acrylic acid mixed gas obtained by vapor-phase oxidation of propylene and/or propane, by reacting the acrylic acid stream in the presence of a mixed metal oxide containing at least Mo and/or Bi (JP 10-218831) or in the presence of a propionic acid reduction metal mixed oxide catalyst containing at least one element selected from the group of Mo and W (EP 2 039 674). These methods either involve a high temperature (300-500° C. in JP 10-218831), or lead to a significant loss of acrylic acid (more than 6% in EP 2 039 674).
Otherwise, the catalytic oxidative dehydrogenation of saturated aldehydes in unsaturated aldehydes is well known in prior art.
For example, Hargis et al, in I&EC product research and development, Vol 5, No. 1, March 1966, pp 72-75 propose to use an oxide of arsenic, antimony or bismuth to convert some saturated aldehydes into the corresponding unsaturated aldehydes. Propionaldehyde is converted into acrolein by using Sb2O4 as oxidant, but only with a selectivity of 62% for a conversion of 5%.
The oxydehydrogenation of saturated aldehydes to unsaturated aldehydes has been also described, more specifically for the production of methacrolein from isobutyraldehyde in U.S. Pat. No. 4,381,411, using iron phosphorous oxide containing at least one promotor. Conversion rates of 100% have been obtained for methacrolein yields ranging between 52% and 80%.
In Kinetics and Catalysis, Vol 44, No. 2, (2003) pp 198-201, isobutyraldehyde has been converted in methacrolein on a iron phosphate catalyst with a conversion of 80% and a selectivity of 82%. Moreover, it has been shown that the addition of a very small amount of molybdenum to Fe—P catalyst enhances the oxidation activity without modifying the high selectivity that originates from iron phosphate. This effect has not been studied in oxy-dehydrogenation of propanal into acrolein, but it has been observed that when the amount of molybdenum is larger that 4% in the catalyst, the selectivity falls, teaching a way to use molybdenum based catalyst.
The document JP 54-046705 discloses a process for preparing unsaturated aldehydes such as acrolein or methacrolein, and carboxylic acids such as acrylic acid or methacrylic acid, by the vapor phase oxidation of C3 and C4 saturated aldehydes such as propanal or isobutyraldehyde, in the presence of catalysts containing Mo, P oxides and one or two elements chosen among Zn, Cu or Ag, supported on a calcined carrier having a specific surface area of at least 10 m2/g. In case of propanal, a yield of 53.1% of acrolein and acrylic acid is obtained at a conversion level of 72.6%, meaning that high conversion of propanal combined with a high selectivity in acrolein is difficult to obtain.
In Journal of Catalysis 195, 360-375 (2000), Ji Hu et al have studied the oxidative dehydrogenation of isobutyraldehyde to methacrolein over P—Mo catalyst. FIGS. 1 and 2 illustrate the effect of temperature on the reaction. They show that the conversion of isobutyraldehyde reaches a maximum of about 95% even at high temperature. It appears that it is difficult to convert more than about 95% of isobutyraldehyde without affecting the selectivity in methacrolein.
In React. Kinet. Catal. Lett. Vol 81, No. 2, 383-391 (2004), the same reaction have been studied over CsPMo catalyst by Cicmanec et al. FIGS. 3 and 4 illustrate the dependence of the conversion of isobutyraldehyde and the yield of methacrolein on contact time. It appears that the conversion is limited to 90% even with high contact time.
Prior art on the whole teaches away to convert saturated aldehydes present at low level in the corresponding unsaturated aldehydes since the man of the art would expect a degradation of the catalyst properties and a decrease of selectivity in unsaturated aldehydes.
In WO09/127,889, a 16.3% acrolein yield was obtained from glycerol at 91.3% conversion with a heteropolyacid containing molybdenum. It shows that heteropolyacids containing molybdenum are bad catalysts for the production of acrolein from glycerol. For that reason, the man of the art is not tempted to use those catalysts to eliminate propanal as an impurity in acrolein.
It is therefore an objective of the present invention to provide a process for selectively removing propanal as an impurity from an acrolein-rich stream without affecting acrolein. As a result, it has unexpectedly been found that the most selective catalysts for propanal elimination in an acrolein-rich stream comprise at least the element molybdenum.
It is another objective of the present invention to provide a process for manufacturing acrolein and/or acrylic acid containing low amount of propanal and/or propionic acid.
It is another objective of the present invention to provide a process for manufacturing acrylic acid from glycerol including a step of selective elimination of propanal, while providing acrylic acid essentially based on a non-fossil natural carbon source and overcoming the drawbacks of the existing catalysts for the dehydration of glycerol.